1. Field of the Invention
The present invention relates to apparatus and method for detecting the position and/or speed of a rotating body (a rotating object) or a moving body (a moving object). The present invention also relates to systems which transmit to a controller an output signal from a sinusoidal wave encoder as a position detector.
2. Description of the Prior Art
An encoder is known which detects the rotational angle of a rotating body or the position of a moving body magnetically or optically. The encoder changes the signal from the rotating body or the moving body to a pulse signal and counts rise or fall edges of the signal to detect the position of the rotating body or moving body.
The apparatus disclosed in a Japanese application Laid-Open JP-A-61-81185 by H. Tamura et al changes encoder signals to pulse signals, and counts the pulse signals to obtain the speed or position of a rotating body or a moving body. The apparatus includes a high-resolution encoder used at low speed and a lower-resolution encoder used at higher speed.
Data on the actual speed is obtained by counting pulses produced for a predetermined sample time or a predetermined number of (two or more) inter-pulse time intervals.
A specific speed detection process will now be described with reference to FIG. 4 in which reference character P denotes a train of pulses obtained from the encoder, and Ts is a sample time which is about 0.5 msec in the particular example. A high accuracy value indicative of the detected speed is obtained by detecting the number of pulses P.sub.n produced for a sample time T.sub.s, and the interval T.sub.d between the pulses P.sub.n, and performing the following division: ##EQU1##
When the rotating body or moving body falls in a low or extremely low speed region, no pulses P.sub.n are produced in a predetermined sample time to thereby render the detection of the speed uncertain.
This process is hereinafter referred to as a pulse detecting system. According to this system, it it impossible to detect a position falling between adjacent pulses even if the number of pulses per rotation and resolution are increased using excellent manufacturing techniques.
For example, a direct drive motor which drives a load using no gears can rotate at an extremely low speed lower than one rotation per minute. According to the pulse detection system, there are no plurality of pulses in a sample time, or there are only a very few pulses, if any, and therefore, stabilized speed control cannot be expected. It is obvious that the use of long sample time increases the number of pulses to be detected in the sample time to thereby enable stabilized control, of course. However, the responsiveness to control is lowered.
The original signal from the encoder generally takes the form of a sine wave signal or a signal similar thereto. In a sine wave signal detection system which uses the analog value of the original signal as it is as a position signal, a superhigh resolution of more than a million pulse per rotation is obtained to thereby enable substantially stepless position detection and to bring about a shortened sample time and rapid control.
On the other hand, since the analog signal is used, the encoder and the controller cannot be connected in an isolated manner through a photocoupler for a signal transmission. Therefore, the controller is likely to be influenced by noise to thereby render it impossible to provide a long transmission distance.